This invention relates to methods of leaching metals from spent hydroprocessing catalysts, particularly methods of enhancing cobalt recovery from spent hydroprocessing catalysts by the use of sulfur dioxide leaching.
One modern development in crude oil processing is the upgrading of metal and sulfur containing feedstocks, e.g, crude oils and residua by hydroprocessing methods. Such upgrading is necessary to convert the heavy feedstock into more valuable, lower boiling fractions and to remove contaminants, particularly metals and sulfur, that can pollute the atmosphere upon combustion.
Crude oils contain various dissolved contaminants, including nickel, vanadium, iron, and sulfur. The lighter fractions are frequently distilled off under atmospheric pressure or a partial vacuum leaving the metals in a high boiling fraction generally called the "residual fraction," or "residua." Residua will generally contain at least 35 ppm metal contaminants, frequently as high as 100 ppm, and in extreme cases, higher than 1000 ppm.
These metals and any sulfur present are removed, thereby upgrading the feedstock, by processing the feedstock, with a catalyst, in the presence of hydrogen. Such catalysts are generally a solid support that contains catalytic metals, generally either molybdenum or tungsten with either nickel or cobalt. As the catalyst is used, metals from the feedstock deposit on its exterior surface and the interior surface of its pores, eventually plugging the pores and reducing the activity of the catalyst to such an extent it does not give the desired product quality. Such catalysts are herein defined as "spent catalysts," and contain catalytic metals, an inorganic support matrix, metals removed from the feedstock, sulfur compounds, and a hydrocarbonaceous residuum.
Recently, the obtainable crude oil is tending to be heavier, forcing refiners to use more hydroprocessing catalysts than heretofore necessary to remove metals and sulfur from the feedstock. A shortage of the valuable catalytic metals, particularly cobalt, is therefore possible. In an effort to recycle both the catalytic metals and the catalyst supports, providing a renewable source of catalytic metals, efforts have been made to extract metals from hydroprocessing catalysts, particularly hydrodesulfurization and hydrodemetalation catalysts.
One general method of leaching hydroprocessing catalysts is disclosed in U.S. Pat. No. 3,567,433. An aqueous ammonia and ammonium salt leach solution is contacted with spent catalyst particles. The conditions of the system were not optimized, resulting in low metals recovery.
Another leaching process is disclosed in Chemical Abstracts, 94:178649x. A spent catalyst, containing aluminum, vanadium, nickel, cobalt, and molybdenum, was leached with ammonia and ammonium salts, at a temperature greater than 110.degree. C. and an oxygen partial pressure of greater than 1 kg/cm.sup.2, for more than 1/2 hour.
Other methods of recovering metals from spent demetalation or desulfurization catalysts are known. U.S. Pat. No. 4,216,118 discloses chlorinating spent catalysts to convert vanadium values to vanadium tetrachloride and nickel values to nickel chloride for recovery by solvent extraction. U.S. Pat. No. 4,145,397 discloses recovery of metals from spent catalysts by roasting at high temperatures and leaching with caustic alkali.
An article in Engineering and Mining Journal, May 1978, page 105, describes a plant to process spent catalysts containing no cobalt by first leaching with sodium hydroxide and then with ammonium carbonate.
Cobalt is a particularly difficult metal to remove from hydroprocessing catalysts by conventional aqueous leaching techniques. Under optimum leaching conditions, an aqueous leach solution of ammonia and an ammonium salt rarely removes more than about 50 percent of the cobalt present on the spent catalyst. It has been discovered that if the spent catalysts are leached by a first aqueous solution of ammonia and ammonium salt and subsequently leached by a second aqueous solution having sulfur dioxide dissolved therein, total recovery of cobalt can be well over 90 percent of the metal present on the initial spent catalyst.